Heart rate (HR) and heart rate variability (HRV) are measurable quantities that help to assess the health and/or fitness of a living subject. For example, HRV measurements may be used in the detection of stress in humans. One device used to measure or obtain these parameters is a photoplethysmograph (PPG), which illuminates skin and detects changes in skin volume by inspecting transmitted or reflected light. Changes in volume caused by skin or blood vessels being enlarged or shrunk by heart beats can be measured to detect the heart beats. The HR and/or HRV can be determined from the detection of heart beats and intervals between heart beats, sometimes referred to as an inter-beat interval (IBI). PPG devices can be relatively compact, and can be portable or can be worn by an individual subject.
When a living subject is in motion, the accuracy of a PPG device is diminished. Noise in the form of motion artifacts is introduced in the sensor readings. Accordingly, it may be difficult to reliably measure or obtain the IBI of a subject in motion using a PPG device. Robust measurement of HRV becomes challenging in such circumstances.
Artifacts introduced by movement can partially be countered by increasing the light output of the light source in the PPG device. High power use of the light source in this manner causes the battery life of these wrist-worn PPG devices to be significantly reduced.
Artifacts may also be countered with the use of accelerometers that can detect motion of the PPG device and provide compensation estimates for signals obtained from the PPG device. However, use of one or more accelerometers to detect motion for motion artifact compensation tends to increase cost, size and power consumption of the overall device.
Another technique that can be used to measure HR and HRV is based on skin conductance (SC). An SC-based device measures electrical characteristics of an area of skin to determine various quantities of interest, including HR and/or HRV. For example, during physical activity, SC is attenuated by the beating of the heart, which permits detection of HR and/or HRV. However, SC devices sometimes encounter accuracy challenges in the absence of physical activity.